Re: [PATCH 2/3] mm, vmscan: Only clear pgdat congested/dirty/writeback state when balanced

From: Vlastimil Babka
Date: Fri Mar 10 2017 - 04:06:55 EST


On 03/09/2017 08:56 AM, Mel Gorman wrote:
> A pgdat tracks if recent reclaim encountered too many dirty, writeback
> or congested pages. The flags control whether kswapd writes pages back
> from reclaim context, tags pages for immediate reclaim when IO completes,
> whether processes block on wait_iff_congested and whether kswapd blocks
> when too many pages marked for immediate reclaim are encountered.
>
> The state is cleared in a check function with side-effects. With the patch
> "mm, vmscan: fix zone balance check in prepare_kswapd_sleep", the timing
> of when the bits get cleared changed. Due to the way the check works,
> it'll clear the bits if ZONE_DMA is balanced for a GFP_DMA allocation
> because it does not account for lowmem reserves properly.
>
> For the simoop workload, kswapd is not stalling when it should due to
> the premature clearing, writing pages from reclaim context like crazy and
> generally being unhelpful.
>
> This patch resets the pgdat bits related to page reclaim only when kswapd
> is going to sleep. The comparison with simoop is then
>
> 4.11.0-rc1 4.11.0-rc1 4.11.0-rc1
> vanilla fixcheck-v2 clear-v2
> Amean p50-Read 21670074.18 ( 0.00%) 20464344.18 ( 5.56%) 19786774.76 ( 8.69%)
> Amean p95-Read 25456267.64 ( 0.00%) 25721423.64 ( -1.04%) 24101956.27 ( 5.32%)
> Amean p99-Read 29369064.73 ( 0.00%) 30174230.76 ( -2.74%) 27691872.71 ( 5.71%)
> Amean p50-Write 1390.30 ( 0.00%) 1395.28 ( -0.36%) 1011.91 ( 27.22%)
> Amean p95-Write 412901.57 ( 0.00%) 37737.74 ( 90.86%) 34874.98 ( 91.55%)
> Amean p99-Write 6668722.09 ( 0.00%) 666489.04 ( 90.01%) 575449.60 ( 91.37%)
> Amean p50-Allocation 78714.31 ( 0.00%) 86286.22 ( -9.62%) 84246.26 ( -7.03%)
> Amean p95-Allocation 175533.51 ( 0.00%) 351812.27 (-100.42%) 400058.43 (-127.91%)
> Amean p99-Allocation 247003.02 ( 0.00%) 6291171.56 (-2447.00%) 10905600.00 (-4315.17%)
>
> Read latency is improved, write latency is mostly improved but allocation
> latency is regressed. kswapd is still reclaiming inefficiently,
> pages are being written back from writeback context and a host of other
> issues. However, given the change, it needed to be spelled out why the
> side-effect was moved.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>